1. Field of the Invention
The present invention relates to an adaptive antenna array system and a method of calculating and controlling weighting coefficient.
Priority is claimed on Japanese Patent Application No. 2003-95364, filed Mar. 31, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
In conventional adaptive antenna array systems, an antenna array is provided that is composed of a plurality of antenna elements, and the received signal of each antenna element is synthesized and output after weighting the signal using a weighting coefficient for controlling the directivity of the antenna array. In addition, a known example of a method of calculating the weighting coefficient involves the use of an adaptive algorithm based on the minimum mean square error (MMSE) to calculate the weighting coefficient using adaptive control so as to minimize the square error between the received signal of each antenna element and a reference signal (refer to, for example, Nobuyoshi Kikuma, ed., “Adaptive Signal Processing by Antenna arrays”, Science Press, Inc., November 1998, p. 13-66). Examples of the aforementioned MMSE-based adaptive algorithm include Least Means Square (LMS) and Recursive Least-Squares (RLS).
In addition, a system has also been studied that reduces the power consumption of the arithmetic processing device (e.g., digital signal processor (DSP)) by reducing the number of calculations for updating the aforementioned weighting coefficient. In this type of system, as is shown, for example, in Japanese Unexamined Patent Application, First Publication No. 2002-77012, by evaluating the propagation path characteristics of a received signal, and controlling the number of updates of the aforementioned weighting coefficient corresponding to the results of this evaluation, the number of calculations for updating the weighting coefficient is known to be reduced. An example of a parameter used to represent propagation path characteristics is the phasing speed of a mobile terminal device. If the phasing speed is slower than a predetermined value, there is judged to be little fluctuation in the weighting coefficient itself, and the weighting coefficient calculated with the previous frame is continued to be used, thereby reducing the number of updates of the weighting coefficient. For example, the number of updates is taken at a ratio of one frame for each of a predetermined number of frames.
In addition, updating processing of the weighting coefficient may be made to not be performed until the error of the weighting coefficient increases to the extent that a cyclic redundancy check (CRC) error or other reception error occurs in the received signal.
However, the aforementioned prior art has the problems indicated below.
In the case of a system that reduces the number of updates of the weighting coefficient based on the phasing speed of a mobile terminal device, since the judgment as to whether or not the number of updates of the weighting coefficient can be reduced is dependent upon empirically obtained conditions, the number of updates is not always reduced corresponding to the fluctuation in the weighting coefficient, thereby resulting in the risk of deterioration of reception quality.
In addition, in the case of a system that updates the weighting coefficient after a reception error is generated, there is the risk of having a serious detrimental effect on reception quality, thereby making this undesirable.
In order to accurately make the weighting coefficient follow fluctuations in the propagation path characteristics of a received signal, it is not desirable to reduce the number of updates of the weighting coefficient. On the other hand, a large number of calculations increases the current consumption of the mobile terminal, thereby having the detrimental effects of increased battery size and shortened battery life.
On the basis of such reasons, there is a need to obtain a suitable weighting coefficient while also reducing the number of calculations required.
In consideration of such circumstances, the object of the present invention is to provide an adaptive antenna array system and weighting coefficient calculation and control method capable of reducing current consumption by reducing the number of calculations required to obtain a suitable weighting coefficient.